This is one of the most famous entries in the Kangin shū (閑吟集, "Quiet recital collection"), a late Muromachi (16th C.) collection of "folk poems":

何せうぞ　くすんで　一期は夢よ　ただ狂へ
Nani shō zo/ kusunde/ ichigo wa yume yo/ tada kurue

Why you gotta act all serious?
Life's a dream -- just go crazy.

This is more or less the theme of the entire collection. It's live-for-the-moment stuff. The poem immediately before this one goes: "I can't even bear to look at serious folks/ with wide-awake faces/ here in this world/ of dreams, of dreams, of dreams!" (くすむ人は見られぬ、夢の／＼／＼世を現顔して). You get the idea.

This kind of thing was very popular in the Muromachi period, especially towards the end when Japan was engulfed in even more civil wars than usual. One wonders what explanation literary historians will find for the exhortations in modern American music to get retarded, hyphy, etc.

Incidentally, you know why it's called the "Quiet recital collection" rather than the "Cavalcade of boisterous revelry"? Because it was compiled by a self-described "lone śramaṇa (桑門)" by the light of the moon in his "quiet abode" (閑居) where he talks to the fireflies. It's all about backstory.